Humidity
Humidity is the water vapour present in the air. It varies from almost 0 to about 4% of the air by volume.
- Absolute humidity: the actual weight of water vapour in a unit volume of air, e.g. grams per cubic metre (g/m³).
- Relative humidity (RH): the vapour present as a percentage of the most the air can hold at that temperature. RH = (actual vapour ÷ vapour at saturation) × 100.
- Saturated air: air holding all the vapour it can (RH = 100%).
- Dew point: the temperature at which air becomes saturated when cooled.
Warm air can hold more vapour. So if air cools without losing vapour, its RH rises. RH is high over oceans and low over continents and deserts.
Evaporation and condensation
Evaporation is water changing to vapour. It needs heat: the latent heat of vaporisation. It is faster when it is hot, the air is dry, and the wind is blowing (wind carries wet air away).
Condensation is vapour changing back to water. It happens when air cools to its dew point, and the vapour needs tiny particles (hygroscopic nuclei like dust, smoke, salt) to sit on. Condensation releases the stored latent heat, which warms the air and powers storms.
Air can be cooled by: rising and expanding (the main way for clouds), touching a cold surface, or mixing with colder air.
Forms of condensation: dew, frost, fog, mist and clouds
- Dew: water drops on cool surfaces like grass and leaves. Best conditions: clear sky, calm air, high RH, cool long nights; dew point above 0 °C.
- Frost: ice crystals formed directly on cold surfaces when the dew point is below 0 °C.
- Fog and mist: a cloud touching the ground, made of tiny drops on dust or smoke. Fog is thick (you see less than 1 km); mist is thinner and wetter. Smoke + fog = smog.
- Clouds: masses of tiny drops or ice crystals high above the ground. Four main types:
- Cirrus: high (8–12 km), thin, feathery, white.
- Cumulus: puffy, like cotton, flat base; fair-weather clouds at 4–7 km.
- Stratus: layered sheets that cover the sky; grey.
- Nimbus: dark, thick rain clouds, low down.
Precipitation and types of rainfall
Precipitation is water falling to the ground after condensation. Forms:
- Rain: liquid drops.
- Snowfall: ice flakes when it is below 0 °C.
- Sleet: frozen raindrops, or rain mixed with snow.
- Hailstones: balls of ice made in tall thunderclouds as drops are carried up and down and gain layers of ice.
Types of rainfall (by what lifts the air)
- Convectional rain: hot ground heats air, which rises, cools and makes cumulonimbus clouds; heavy afternoon showers with thunder. Common in equatorial areas and in summer.
- Orographic (relief) rain: moist wind meets a mountain and is forced up. The windward side gets heavy rain; the leeward side gets little: the rain-shadow area.
- Cyclonic (frontal) rain: warm air is lifted over cold air at a front or in a cyclone.
World distribution of rainfall
- Rain generally decreases from the equator to the poles.
- Over 200 cm: equatorial belt and windward sides of mountains on coasts where moist winds blow (like the monsoon coasts).
- Low (under 50 cm): subtropical deserts near 30° (sinking dry air), the interiors of continents far from the sea, rain-shadow areas, and polar regions.
- Coasts get more than interiors. Between 35° and 40° N/S, the eastern coasts get more rain; beyond 45° the western coasts get more (westerlies).
- Oceans get more than land.
- Seasonal pattern: some places get rain all year (equatorial), some mainly in summer (monsoon lands), some mainly in winter (Mediterranean-type lands).
Try it at home
Take a steel glass of ice water out on a humid day. In a minute tiny drops form on the outside. The glass cooled the air next to it below its dew point: condensation. Breathe on a cold mirror: the fog on it is the same thing. Leave a flat plate outside on a clear, calm night and check it at dawn for dew.
Key formulas and definitions
- Relative humidity (%) = (actual vapour ÷ vapour air can hold at that temperature) × 100
- Saturated air: RH = 100%; dew point = temperature at which air becomes saturated
- Evaporation takes latent heat; condensation releases it
- Clouds: Cirrus (high, feathery), Cumulus (puffy), Stratus (layers), Nimbus (rain)
- Rainfall types: Convectional, Orographic (windward wet, leeward rain-shadow), Cyclonic/Frontal
Worked examples
1. Air at 30 °C holds 15 g/m³ of vapour. It could hold 30 g/m³ at that temperature. Find its relative humidity.
RH = 15 ÷ 30 × 100 = 50%.
2. The same air cools to 20 °C, where it can hold only 17 g/m³. What is the new RH? Will dew form?
RH = 15 ÷ 17 × 100 ≈ 88%. Not yet 100%, so no dew yet; it has to cool further to its dew point.
3. Air has 12 g/m³ of vapour. At 10 °C, the most it can hold is 9 g/m³. What happens if it cools to 10 °C?
It can hold only 9 g/m³, so 12 − 9 = 3 g of water per m³ must condense into drops (dew, fog or cloud).
4. Why does Mumbai get far more rain than Pune in the monsoon?
Moist south-west winds rise over the Western Ghats and drop most of their rain on the windward (Mumbai) side. Air sinking on the leeward (Pune) side warms and dries: rain-shadow.
5. Why are there few clouds over the Sahara even though it is very hot?
It lies under the subtropical high where air sinks. Sinking air warms and its RH falls, so it cannot reach dew point, and there is little moisture to begin with.
Common mistakes
- Thinking relative humidity is the amount of water in air. It is how full the air is (a percentage), not the amount.
- Mixing dew and frost: frost forms when the dew point is below 0 °C, not by freezing of dew.
- Thinking fog and clouds are different things. Fog is a cloud at ground level.
- Saying the leeward side of a mountain gets the most rain. It is the windward side; the leeward side is the rain-shadow.